Hello
Should the failure warning switch under the Lockheed master cylinder just unscrew? I can turn the hexagon body about 15 degrees but I've a nasty feeling that the thread (if that's what it is) isn't following on....
Alex.

Well as expected the switch body gave up the fight and left me with this -

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Removed the plunger and chaindrilled the plastic and dug it out -

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and removed the operating piston. Which was really really clogged with cr++! This is how the new switch sits in position relative to the piston - no ramps, just square faces so I don't see how the piston can move the plunger at all, unless hydraulic pressure comes into play???

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The new switch is part No AAU2454 from MGB Hive and fits just fine. However it seems to operate differently from the original, the original had both its terminals connected together (to provide continuity to the pad sensors) and I think when the piston moved it shorted them to earth (to light the warning lamp). But the MGB switch's terminals are normally closed and open when the plunger moves. No short to earth that I can see, so I've no great hope that it'll give the desired results back on the car. Unless someone knows different?

Some more information has come out of the Internet service.
It seems that the white switch that I have is wrong, it's designed to operate with a shuttle valve that drops the plunger rather than lifts it. The correct one, I hope, is part No AAU1752A and it's black rather than white. This one's plunger doesn't move at all, it's supposed to short out against the square shoulders on the shuttle piston when the piston moves along. Strange but true, apparently!
I've ordered a black one, more news when it arrives.

is this the switch we remove if fitting the replacement master cylinders you supplied, onto a small sports? If so, I may still have one on the shelf.

I'm surprised there's a need to remove it, I'd have thought you'd just leave it in position but not connected?
If it is the "Corky type" IIRC didn't they have a brass switch? Not sure if that would be the same thread/operating method?

....Roger

RSSOC member (since 1982)
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"Condition can be bought at any time; Originality, once lost, is gone forever" - Doug Nye

I did find a replacement switch a few years ago. As I said it my earlier post it was either MGB or LDV/Sherpa. In my limited experience I found that there is more than one type of original MC. So there may not be one switch to cover all types of MC.

By older I mean original equipment. About seven or eight years ago replacement MCs were virtually unobtainable, so with the help of a parts supplier and a bit of experimentation to get the fitment right, I developed a replacement. These have the brass switch that Jim referred to. There are other differences too, meaning that an original rebuild kit won’t fit them.
AJL bought the remains of the last batch and has them for sale in his web shop.

The brass fail switches are basically a contact insulated from the body of the MC. On brake fail the shuttle piston makes contact with it, and you have your earth. Maybe a simple modification of your MGB switch would have the same effect.

From your photo, it would appear that the pin/plunger sits in between the two steps of your shuttle piston. Is this correct ? Should the pin be permenantly pushed down by one of the steps, and it fails open ? Or do the sides of the steps make contact with the pin/plunger to make a contact with earth ? There are no ramps (other than towards the end) on the shuttle, for it to move smoothly over the pin/plunger and push it down, so is it normally located towards the end. Just trying to get my head around the operation, as it's a few years since I last stripped one down.

Steve
It sounds like your brass switch was spot on, its job is to provide a fixed live contact which can only be earthed by the shuttle valve moving one way or t'other and hitting the contact. Obviously the plastic body does the insulating in the standard switch. The two plug contacts are only to provide daisy-chaining on to the pad warning wires.
The operation is as your second theory, the 'switch' contact is rigid, doesn't go up and down but sits and waits for the step on the valve piston to come along. The piston normally sits central to the contact and is pushed either way by unequal fluid pressure. There should be two O-rings on the piston, one's missing from the photo because I hadn't spotted it still lurking in the cylinder!!
There is apparently another version of the Lockheed fail switch which is operated by ramps on the piston and does the switching inside its body. Maybe that's the MGB one you were thinking of.